Field of the Invention
The invention relates to a high-temperature fuel cell system and to a method for operating the fuel cell system.
It is known that water molecules are decomposed by electrical current into hydrogen and oxygen during the electrolysis of water. In a fuel cell, that process takes place in the opposite direction. When hydrogen and oxygen are electrochemically combined to form water, electric current is produced, with high efficiency and without any emission of hazardous materials or carbon dioxide, if pure hydrogen is used as a combustion gas. Even with technical combustion gases, for example natural gas or coal gas, and with air or air enriched with O.sub.2 instead of pure oxygen, a fuel cell produces considerably fewer hazardous materials and less CO.sub.2 than other energy producers that operate with fossil energy sources. The technical implementation of the principle of the fuel cell has led to widely different solutions, to be precise with various types of electrolytes and with operating temperatures T.sub.0 between 80.degree. C. and 1000.degree. C. Fuel cells are divided on the basis of their operating temperature T.sub.0 into low, medium and high-temperature fuel cells, which in turn are distinguished by various technical configurations.
In the case of the high-temperature fuel cell (Solid Oxide Fuel Cell, SOFC), for example, natural gas is used as the primary energy source. The very compact structure allows a power density of 1 MW/m.sup.3. The operating temperatures T.sub.0 are above 900.degree. C.
As a rule, a fuel cell block, which is also called a "stack" in the specialist literature, is composed of a large number of fuel cells of planar construction that are stacked one above the other.
In order to operate a fuel cell system including at least one fuel cell block at a high, constant operating temperature T.sub.0 of more than 900.degree. C., for example, it must be supplied with heat before operation in order to reach the operating temperature T.sub.0, and it must be supplied with heat in order to maintain the required operating temperature T.sub.0 during brief breaks in operation. Present-day fuel cell blocks have relatively low power levels and have dimensions on a laboratory scale. A furnace is used to raise them to the operating temperature T.sub.0 of about 600.degree. C. for MCFC (Molten Carbonate Fuel Cell) or about 950.degree. C. for SOFC, and they are operated in the furnace. That solution is impractical for fuel cell blocks with higher power levels and larger dimensions.
A symposium report entitled "A Study for a 200 kWe-System for Power and Heat", by M. R. Taylor and D. S. Beishon, in the "First European Solid Oxide Fuel Cell Forum", Lucerne 1994, pages 849 to 864, discloses a method which passes power-plant gas through the fuel cell block in order to heat it. That method is disadvantageous since the power-plant gas pollutes or damages the fuel cells of which the fuel cell block is composed.
German Published, Non-Prosecuted Patent Application DE 42 23 291 A1 discloses a fuel cell system unit which includes a cell stack composed of a large number of individual fuel cells. When operation starts, an apparatus disposed outside the fuel cell system unit for raising the temperature heats water passing through the fuel cell system unit, as a result of which the individual fuel cells are preheated to a predetermined temperature.
German Published, Non-Prosecuted Patent Application DE 40 37 970 A1 discloses a method in which operating material for the fuel cell stack is heated by hot exhaust gas from a fuel cell stack. Such a method is also disclosed in Published European Patent Application 0 654 838 A1.